In rotary drilling of boreholes, the usual practice is to use a rotary drilling bit which is secured to the lower end of a drill string. Dulling of the drill bit requires retrieving the entire drill string, replacing the dull bit with a new or rebuilt bit, and then relowering the entire drill string into the borehole. This is a relatively slow and costly operation, due in large part to the time consumed in uncoupling and subsequently recoupling sections of the drill pipe. Moreover, the time and costs increase as the depth of the hole increases. The costs become even greater in offshore drilling, in view of the very high hourly rates involved in operating offshore drilling platforms and drilling vessels.
It has been proposed to avoid the expense of raising and lowering an entire drill string by providing a drill bit of a type that can be installed and retrieved directly through the drill string. When the drill bit is dull, the bit would be replaced without withdrawing the entire drill string. Attempts at this approach, however, have not been successful since the drill bit cutters have been relatively small, thereby placing limitations on the diameter of the hole which they can drill effectively. The small size of the bits is due, in part, to the fact that the inside diameter of the drill string is relatively small. The largest inner diameter of a conventional drill string is around 4 inches.
The prior art has described several types of retrievable drill bits. None of these bits, however, have satisfactorily dealt with the operating conditions encountered while drilling a borehole. These conditions include tremendous forces which act on a bit while drilling. These forces may vary depending on bit surface area, type of formation being drilled, friction and heat. In improperly designed bits, these forces can be transferred onto members which cannot withstand the forces. The structural design of drill bits is therefore extremely important; otherwise, large drilling load forces may be centered on a few small parts which are vital to keeping the bits from breaking off or otherwise becoming inserviceable. This has been a particular problem with retrievable bits of the prior art.
Drilling rig operators shy away from drill bits which may break off in a borehole, because of the problems and costs encountered in fishing small pieces of drill bit out of the borehole. They also shy away from bits which are unreliable in their performance. Retractable drill bits in the prior art have failed to produce a reliable mechanical method of locking a drill bit into its normal operating position and thereafter retrieving the bit. Methods of retrieving and locking retrievable drill bits must be reliable and both the methods and the bits must be able to withstand the vibrations, forces and heat that are encountered in the drilling environment.